There are a number of different types of radiation imaging systems currently in use in the field of nuclear medicine. The most common type of systems, known as Anger gamma ray cameras, comprise a collimator, a scintillation crystal which emits photons when struck by gamma rays, and an array of photomultipliers tubes and associated electronics which detect the photons. Anger cameras have been used for many years for various applications including the imaging of gamma rays emitted by a radionuclide put into a patient. The sensitivity and resolution of Anger gamma cameras are determined by a number of factors, such as radionuclide used, patient geometric factors (distance, attenuation, scatter, distribution), camera collimator, detecting scintillation crystal, electronics/circuitry and computer/display. Most gamma cameras are designed to give the best combination of sensitivity and resolution at about 140 keV, the energy of the major gamma ray photon coming from technetium-99m (99mTc, .sup.99m Tc, Tc-99m), the commonest radionuclide presently used in nuclear medicine.
Various efforts have been made to improve the sensitivity and resolution of Anger gamma cameras. Camera sensitivity can be improved by the use of thicker detection crystals, scintillation crystal materials with greater stopping power, and more sensitive collimators and photomultiplier tubes (PMTs). However, the use of thicker crystals and high sensitivity collimators results in loss of resolution. Conversely, improved resolution of the system in prior art Anger designs results in loss of sensitivity. Efforts at improved resolution are usually geared towards using a thinner crystal, higher resolution collimator (with smaller holes packed closer together) or electronics/computer "acceptance" of summation energy detected (usually called the "Z" peak) within a narrower acceptance range (in an attempt to reduce acceptance of scattered photons). However, the sensitivity and resolution of Anger cameras remain poor, especially for higher energy gamma rays. The present best intrinsic resolution of Anger gamma cameras for 99m-Tc is about 3 mm.
Other types of radiation imaging cameras, such as positron emission tomography (PET) cameras, are designed for a specific use and cannot be used for planar imaging or imagining other radionuclides. PET cameras use fixed-ring, thick bismuth germinate crystals, and are very expensive. Recent "hybrid" gamma cameras use thick crystals, very heavy (and expensive) collimators and coincidence circuitry to image positron emitters, but the sensitivity of such cameras is at best 10% that of a dedicated PET system and the resolution at least twice as bad.
There is a lower and upper limit to how much radioactive material can be given to a patient (injected, inhaled, eaten, etc.) in order to see, by gamma camera, how it is handled by the body. The lower limit is determined by the sensitivity of the gamma camera and by the amount of interfering background radiation, and must be great enough to yield acceptable diagnostic imaging or information. The maximal amount is determined, for one, by radiation safety, which specifies that only the minimal amount be used that gives acceptable diagnostic imaging or information.
However, there are other reasons for limiting the maximum amount of radioactive material given to patients. While the gamma camera system (crystal, electronic circuitry and computer) is busy "registering" an event (scintillation in the crystal, signifying interaction with a gamma ray), it is incapable (absolutely or relatively) of responding to another event. This "dead time" for most gamma cameras is of the order of or greater than a micro-second. For higher amounts of radioactivity the proportion of dead time to mean time between events rises in a non-linear fashion. Many electronic systems are also "saturable", i.e. beyond a certain event rate they simple do not respond at all. Any attempt at quantitating amounts of radioactivity present in certain areas in relation to others gives rise to dead time errors which are very large for higher amounts of radioactivity. Also, events (gamma ray interactions producing crystal scintillations) that occur very close in time give rise to a summation scintillation whose total energy (Z-pulse) is well outside the acceptance window and is rejected by the computer as not coming from the gamma ray of interest. This coincidence pile-up, like dead time, also effectively reduces detected count rate in comparison with flux.
Limiting radioactivity to avoid coincidence and dead time errors gives rise to increased statistical errors since radioactivity obeys Poisson statistics. The standard deviation in counts for Poisson statistics is the square root of the counts observed (in any region or time interval). Minimizing quantitation errors therefore requires balancing the effects of too low a count rate against those of too high a count rate. Prior art Anger gamma camera imaging in patients introduces quantitation errors of at least 5%, under the best circumstances. Much higher count rate capabilities would be helpful, since, with many radionuclides, the radiation safety limits are far from being reached.
The collimator is often a major factor affecting resolution in prior art Anger gamma camera imaging, while the crystal is often the major factor reducing sensitivity, although both collimator and crystal influence sensitivity and resolution greatly. Dead time and coincidence losses are mostly due to the large crystals used in Anger gamma cameras (up to about 500 mm) and the fact that each PMT receives scintillation light from the entire crystal.
There is accordingly a need for an improved gamma camera having better resolution, sensitivity, and linearity of response.